G. C. Montanari et al.: Charging Properties and Time-temperature Stability of Innovative Polymeric Cellular Ferroelectrets 1070-9878/07/$25.00 © 2007 IEEE 238 Charging Properties and Time-temperature Stability of Innovative Polymeric Cellular Ferroelectrets G.C. Montanari, D. Fabiani DIE-LIMAT, University of Bologna Viale Risorgimento 2 40136, Bologna, Italy F. Ciani TechImp S.p.A. Via Toscana 11/C 40069, Zola Predosa, Bologna, Italy A. Motori DICASM, University of Bologna Viale Risorgimento 2 40136, Bologna, Italy M. Paajanen VTT Technical Research Centre of Finland P.O. Box 1607, FIN-33101 Tampere, Finland R. Gerhard-Multhaupt and M. Wegener Department of Physics, University of Potsdam Am Neuen Palais 10 14469 Potsdam, Germany ABSTRACT After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics. Index Terms — Ferroelectric materials, partial discharges, piezoelectric films, space charge Manuscript received on 1 February 2006, in final form 21 July 2006.